The present invention relates to a refractory used for glass tank furnaces, in particular, to a porous high-alumina fused cast refractory suitable for upper structures of glass tank furnaces, and a method of its production.
Fused cast refractories are provided by melting formulated refractory raw materials in an electric arc furnace completely, pouring the resulting melt into casting molds having predetermined configurations (casting), and solidifying the melt by cooling to ordinary temperature, in many cases, with heat insulation. It is widely known that fused cast refractories are denser and more corrosion-resistant than fired and unfired bonded refractories.
Among these fused cast refractories, high-Al2O3 fused cast refractories have suitably been used mainly as glass tank furnace refractories. For example, high alumina fused cast refractories mainly composed of xcex1-Al2O3 crystals and xcex2-Al2O3 crystals are frequently used at portions of glass tank furnaces which contact with molten glass and have such dense structures that they have porosities of 4% and less, provided that pores called shrinkage cavities inevitably formed during the cooling step after casting are ignored.
Therefore, improvements of high-Al2O3 fused cast refractories have been focused on densification to minimum porosities with the aim of increasing corrosion resistance against glass.
In recent years, the application of the technique of oxygen burning to glass tank furnaces has generated a new demand on glass tank furnace refractories. Namely, though conventional glass tank furnaces usually use silica bricks having bulk specific gravities of about 2 for ceilings and other upper structures (such as crowns), there is a problem that high concentrations of alkali vapor in glass tank furnaces utilizing the technique of oxygen burning erodes silica bricks considerably. As a countermeasure, use of high-alumina fused cast refractories excellent in corrosion resistance against alkali vapor for these upper structures is considered. Conventional high-alumina fused cast refractories are grouped into two classes: those called void-free which are dense residues of refractories obtained by cutting off shrinkage cavities, and so-called regular casts, which partly contain shrinkage cavities.
It is unadvisable to use void-free high-alumina fused cast refractories for upper structures of glass tank furnaces because such low-porosity refractories having higher bulk specific gravities than silica bricks are heavy in weight and require upper structure supports having high mechanical strength. Another disadvantage of them is their poor thermal shock resistance due to their dense structures.
On the other hand, although regular cast high-alumina fused cast refractories containing shrinkage cavities have low bulk specific gravities, a problem that occurs is that cracks form along the border of the shrinkage cavities because of the great difference in physical properties across the border during the operation of the furnace.
Namely, conventional high-alumina fused cast refractories are advantageous in view of corrosion resistance against glass by virtue of their low porosity and denseness but their high bulk specific gravities is disadvantageous to their use for parts which do not require so much corrosion resistance such as upper structures in view of structural strength and cost.
Meanwhile, increases in the porosities of cast refractories have been attempted. For example, JP-A-59-88360 proposes a porous high-alumina fused case refractory having a porosity of at least 20%. Because the proposed refractory has an alkali metal oxide content as low as 0.25% or below, the porous high-alumina fused cast refractory is composed predominantely of xcex1-Al2O3 crystals. However, xcex1-Al2O3 crystals readily become xcex2-Al2O3 crystals through reaction with alkali vapor while expanding in volume to form a brittle structure. Therefore, the proposed porous high-alumina fused cast refractory does not have enough corrosion resistance for use in the upper structure of glass tank furnaces.
JP-A-3-208869 proposes the use of a foaming agent such as a metal, carbon and a carbide to form pores. The use of a foaming agent has a problem in the production process because the vigorous foaming reaction between a foaming agent and a melt which involves generation of carbon dioxide or the like makes it difficult to control the melting.
The object of the present invention is to provide a porous high-alumina fused cast refractory which has sufficient corrosion resistance against an alkali vapor or the like, is light in weight and has excellent thermal shock resistance and a method of producing it.
The present invention provides a porous high-alumina fused cast refractory comprising from 94 to 98 mass % of Al2O3, from 1 to 6 mass %, in total, of Na2O and/or K2O as chemical components, which is mainly composed of xcex1-Al2O3 crystals and xcex2-Al2O3 crystals, has pores dispersed in it and has a porosity of from 5 to 30%.
The present invention also provides a method of producing a porous high-alumina fused cast refractory comprises from 94 to 98 mass % of Al2O3 from 1 to 6 mass %, in total, of Na2O and/or K2O as chemical components, which is mainly composed of xcex1-Al2O3 crystals and xcex2-Al2O3 crystals, has pores dispersed in it and has a porosity of from 5 to 30%, which comprises blowing a gas, especially a gas containing oxygen, into a molten refractory material, casting and slowly cooling the refractory material to form pores in it dispersedly.
The porous high-alumina fused cast refractory of the present invention (hereinafter referred to as the present cast refractory) comprises from 94 to 98 mass % (hereinafter abbreviated simply as %) of Al2O3, from 1 to 6%, in total, of Na2O and/or K2O (hereinafter referred to as alkali metal oxides) as chemical components.
If Al2O3 exceeded 98% or the alkali metal oxides were less than 1%, the refractory would be mainly composed of xcex1-Al2O3 crystals (corundum crystals, hereinafter referred to as xcex1-crystals) alone, which readily turn into xcex2-Al2O3 crystals (R2Oxc2x7nAl2O3, wherein R is Na or K, and n is a real number around 11, herein after referred to as xcex2-crystals) upon contact with an alkali vapor while expanding in volume when used for upper structures of a glass tank furnace, and the corrosion resistance would become inadequate due to the resulting structural embrittlement.
On the other hand, if Al2O3 were 94% or less or the alkali metal oxides exceeded 6%, the present cast refractory would be mainly composed of xcex2-crystals alone and have such a low compressive strength as 30 MPa or below, and use of the present cast refractory for upper structures of a glass furnace would make a problem in view of mechanical strength. It is preferred that Al2O3 is from 94.5 to 96.5%, and the alkali metal oxides are from 2.5 to 4.5%.
The present cast refractory preferably comprises SiO2 as another component to form a matrix glass phase. The matrix glass phase helps formation of a crack-free refractory by relaxing strain stress which occurs during the annealing. The SiO2 content is preferably from 0.3 to 1.5%, particularly from 0.5 to 1.0%.
The present cast refractory is mainly composed of xcex2-crystals and xcex2-crystals. In addition to xcex1-crystals and xcex2-crystals, the present cast refractory comprises a matrix glass phase comprising SiO2, R2O and CaO as main components (hereinafter the present matrix glass phase) and pores and has such a structure that the matrix glass phase fills gaps between the crystals, and pores are dispersed between the xcex1-crystals, xcex2-crystals and the present matrix glass phase. It is preferred that pores are dispersed uniformly because the durability of the refractory increases with the uniformity of pore dispersion.
In the present invention, with respect to the mass ratio of xcex1-crystals to xcex2-crystals, the preferable ratio of xcex1-crystals/(xcex1-crystals+xcex2-crystals) is from 30 to 70%. It is unfavorable that the mass ratio exceeds 70% because the xcex1-crystals readily turns into xcex2-crystals by reacting with an alkali vapor, and the accompanying volume expansion leads to embrittlement. It is also unfavorable that the mass ratio is less than 30%, because xcex2-crystals turn into xcex1-crystals in turn, and the accompanying volume shrinkage leads to structural embrittlement. The ratio of xcex1-crystals to xcex2-crystals can be controlled by adjusting the R2O content.
In the present cast refractory, pores are formed dispersedly, and the porosity is from 5 to 30%. In the present specification, the porosity is measured with the after removal of shrinkage cavities from the refractory. If the porosity is less than 5%, a porous high-alumina fused cast refractory which is light in weight and excellent in thermal shock resistance can not be obtained, and if the porosity exceeds 30%, the corrosion resistance against an alkali vapor and float components and the strength are insufficient. The porosity is preferably within the range of from 7 to 25%. The porosity (%) is calculated as porosity=(1-(d2/d1))xc3x97100 from the true specific gravity d1 and the bulk specific gravity d2.
In the present invention, it is preferred that at least 80%, preferably at least 90%, of the pores formed dispersedly in it have diameters of from 1 xcexcm to 3 mm because pore are formed without remarkable deterioration in strength while sufficient corrosion resistance is secured. Though pores can have various shapes, because most pores are oval, the size of a pore means the average of the longer diameter and the shorter diameter in the present specification.
The present cast refractory is obtainable like ordinary fused cast refractories by formulating a refractory material of a predetermined composition, putting the refractory material in an electric furnace at a high temperature of at least 2000xc2x0 C. until the refractory material melts completely, pouring the resulting melt into a casting mold having a predetermined shape by casting and slowly cooling the melt, but is characterized in that a large amount of a gas is blown into the melt before casting. Blowing of a gas into a melt allows favorable formation of pores in the refractory and production of a cast refractory having a much smaller amount of shrinkage cavities.
In the present invention, a gas is blown between fusion of the refractory material and casting, preferably while the refractory material is completely molten. A preferable way to blow a gas is to blow a large amount of a high temperature gas through a ceramic or metal tube inserted in the melt. Appropriate control of the kind of the gas, the blow time and the amount of the blow gas allows formation of a given amount of desired pores in the refractory.
In the present invention, though the mechanism of pore formation is unclear, but it is supposed that the high temperature blow gas dissolved in the melt in a supersaturated state is released as the solubility decreases upon cooling. Therefore, oxygen or an oxygen-containing gas containing at least 20 vol % of oxygen such as air is preferable as the blow gas because improvement of porosity is great in proportion of the amount of the blow gas by virtue of its low solubility at low temperature.
It is preferred that the oxygen-containing gas is blown in an amount of from 0.5 to 2.0 (L/1 kg melt) for a few minutes just before casting. A longer blow time is preferable in the case of a gas having a low oxygen content.